Ethylene and propylene are important chemicals for use in the production of other useful materials, such as polyethylene and polypropylene. Polyethylene and polypropylene are two of the most common plastics found in use today and have a wide variety of uses for, for example, a material for fabrication and as a material for packaging. Other uses for ethylene and propylene include the production of vinyl chloride, ethylene oxide, ethylbenzene and alcohol.
The great bulk of the ethylene consumed in the production of the plastics and petrochemicals such as polyethylene is produced by the thermal cracking of higher molecular weight hydrocarbons. Steam is usually mixed with the feed stream to the cracking reactor to reduce the hydrocarbon partial pressure and enhance olefin yield and to reduce the formation and deposition of carbonaceous material in the cracking reactors. The process is therefore often referred to a steam cracking or pyrolysis.
The composition of the feed to the steam cracking reactor affects the results. A fundamental basis of this is the propensity of some hydrocarbons to crack more easily than others. The normal ranking of tendency of the hydrocarbons to crack to ethylene is normally given as: normal paraffins; iso-paraffins; olefins; naphthenes; and, aromatics. Benzene and other aromatics are particularly resistant to steam cracking and undesirable as cracking feed stocks, with only the alkyl side chains being cracked to produce the desired product.
The feed stream to a steam cracking unit can be quite diverse and can be chosen from a variety of petroleum fractions. The feed stream to the subject process preferably has a boiling point range falling within the naphtha boiling point range or about 36 to 205° C. It is preferred that the feed stream does not contain appreciable amounts, e.g. more than 5 mole %, of C12 hydrocarbons. A representative feed stream to the subject process is a C5-C11 fraction produced by fractional distillation of a hydrotreated petroleum fraction. Hydrotreating is desired to reduce the sulfur and nitrogen content of the feed down to acceptable levels. A second representative feed is a similar fraction comprising C5 through C9 hydrocarbons.
The feed to a steam cracking unit is also normally a mixture of hydrocarbons varying both by type of hydrocarbon and carbon number. This variety results in it being very difficult to separate less desirable feed components, such as naphthenes and aromatics, from the feed stream by fractional distillation. The hydrocarbons that are not the normal paraffins can be removed by solvent extraction or adsorption. These hydrocarbons can be upgraded to improve the feedstock to the steam cracking unit.
One way to upgrade these hydrocarbons is to pass the non-normal paraffins to an isomerization zone. In the isomerization zone, the non-normal paraffins are converted, in the presence of a catalyst, into normal paraffins.
Based upon current designs, conversion of iC5 hydrocarbons and iC6 hydrocarbons to normal paraffins in an isomerization zone is limited, by equilibrium conditions, to about 25% and 13% per pass, respectively. Based upon typical processing conditions, full conversion of the iso-paraffins entails large recycle streams, large fractionation columns, and large utility costs. The per pass conversion rates can be increased for example, by increasing the temperature of the isomerization zone, by lowering the liquid hourly space velocities (LHSV), or both, which leads to the cracking of some of the paraffins to lighter C4− hydrocarbons. The cracking reactions can lead to the production of undesired low-value methane.
Furthermore, it has been observed that, within the isomerization zone, the C4 hydrocarbon isomerization activity is suppressed by C5+ hydrocarbon concentrations greater than about 3.0 wt %, and C6+ hydrocarbon concentrations greater than about 0.1 wt %, and C7+ hydrocarbon concentrations greater than about 0.001 wt %. Thus, C4 hydrocarbon isomerization is conducted in a separate process and is not combined with the isomerization of C5 and C6 light naphtha streams.
It would be desirable to have an isomerization process that allows for iC4, iC5, and iC6 hydrocarbons to be efficiently and effectively converted to normal paraffins in the same isomerization zone.